I thought the exact same thing. Then I realized what my time is worth. Sourcing insert material, precision cutting, correct glue etc etc. You easily could spend a couple hours in actual time and labor, plus materials.
My time is worth WAY more than $40 an hour.
Knowing these factors, I feel these ARE PRICED correctly, and I am quite sure the DELTA marketing people agree, that is why they are this price...
And I also agree, these are MANDATORY. Drive you insane without them...
However, I would never stop a man from a learning exercise, so have at it if you feel you need to make your own.